There are many industrial and commercial applications that utilize electrical motors to produce repetitive axial motions. The electrical motor's rotary motion can be converted into axial motion by any number of mechanisms such as cams, cranks, scotch yokes, or cable drums just to name a few. In any such application, the electrical power requirement of the motor is inherently variable and is cyclically locked to the repetitive axial motion. The motor power in these applications varies both due to inertial effects (the need to accelerate and decelerate the axially moving components of the system and the need to accelerate and decelerate the rotationally moving components of the system) and due to the work effects (changes in the work performed by the axially moving components as a function of their axial position and velocity). The magnitude of the motor power variation with time can be many times the average power requirement of the motor. Both the inertial effects and the work effects can cause the motor to function as a generator which produces electrical power at various times in the system's cyclical motion.
An elevator is one well-known example of an electrical motor producing axial motion wherein the motor's electrical power requirements vary with the passenger load, the axial velocity of the elevator and the axial acceleration/deceleration of the elevator. Deliberate deceleration or braking can be achieved by recovering the excess energy in the elevator's mechanical system (e.g. during the descent of a heavily loaded elevator) utilizing regeneration to convert that mechanical energy into electrical energy which can go back into an electrical distribution system.
Another example of a motor producing repetitive axial motion is a pump-jack type oil well. Also known as a walking beam (a large beam arranged in teeter totter fashion) or a walking-horse oil well, the pump-jack oil well generally including a walking beam suitably journaled and supported in an overhanging relationship to the oil well borehole so that a string of rods (as long as two miles) can be attached to the reciprocating end of the walking beam with the bottom end of the rods attached to a lift pump chamber at the bottom of the bore hole. A suitable driving means, such as an electrical motor or internal combustion engine, is connected to a speed reduction unit which drives a crank which in turn is interconnected to the other end of the walking beam by a pitman.
Conventionally, pump-jack oil wells utilize an induction motor powered by constant frequency, three-phase electrical power from a utility grid. The pump-jack pumping cycle varies the induction motor's speed only slightly as allowed by plus or minus a few percent of motor slip. However, the induction motor power typically varies over the pumping cycle by about four (4) times the average motor power level. At two (2) points in the pumping cycle, the motor power requirement peaks and at two (2) other points, the motor power requirements are at a minimum. Typically, at one of these minimum power requirement points in the pumping cycle, the induction motor extracts enough kinetic energy and/or work from the moving masses of the well to be able to function as a generator and produce electrical power which must be absorbed by the utility grid.
Whether the pump-jack oil well is driven by an induction motor or by an internal combustion engine, there is excess mechanical energy at some point(s) in the pumping cycle which must be absorbed to prevent excessive velocity induced stresses in the pump-jack oil well moving parts. When a pump-jack oil well is powered by an internal combustion engine, engine compression is the means by which this energy is dissipated (compression losses) while in the normal utility grid powered induction motor system, the induction motor is periodically driven at overspeed causing it to return power to the utility grid.
When a pump-jack type oil well is powered by constant frequency electrical power from a utility grid or a conventionally controlled turbogenerator, the oil extraction pumping rate may not be sufficient to keep up with the rate at which oil seeps into the well. In this case, potential oil production and revenues may be lost. Alternately, the oil extraction pumping rate may be greater than the rate at which oil seeps into the well. In this case, the oil well may waste power when no oil is being pumped or it may be necessary to shut down the oil well for a period of time to allow more oil to seep into the well.
For the reasons stated above, what is needed is an improved technique for providing power [generally] suitable for pump-jack oil well systems.